I use alternately Spotify and Last.fm to listen to music. When streaming via Airfoil to my Hi-Fi, I found that using "Soundflower" made it easier, because I could set Last.fm's output to "Soundflower (2ch)" (in Preferences > Radio) and capture "Soundflower (2ch)" in Airfoil, instead of capturing Last.fm directly and having to switch every time I switched between Spotify and Last.fm

When done streaming with Airfoil, Soundflowerbed can be used to toggle redirection of the soundflower stream to the built-in output.